The present invention relates generally to a support for a splice in an elongate element, and to a method of applying such a support.
The invention finds particular, but not exclusive, utility in connection with splicing of optical fibres. Optical fibres comprise an inner core of transparent material surrounded by an inner layer of a material having a close but different refractive index and in turn surrounded by a protective cladding layer. When it is necessary to join two optical fibres end-to-end these are spliced by baring the inner core of the end portions of the fibres to be spliced, cleaving the ends and placing them in a closely juxtaposed relationship using equipment capable of ensuring optical alignment of the fibres, and then applying localised heating, for example by appropriately positioned electrodes generating a spark at the junction region, whereby to fuse the end portions of the fiber to form a splice.
Although a spliced region of optical fiber is coherent as far as its optical properties are concerned, the splicing operation leaves the fiber weaker at the junction than in the remaining portion of the fiber, and the fact that it has been bared in order to expose the core for formation of the splice results in a need for additional support to protect the splice against damage due to physical movement. This can be achieved by the application by a splice support sleeve which is preliminarily fitted over one of the fibres to be spliced, and then brought into position over the splice region after the fusion of the junction region has been effected. Splice support sleeves are known comprising a cylindrical body of a dimensionally heat recoverable material the physical dimensions of which can change significantly with changes in temperature (known as xe2x80x9cheat-shrinkxe2x80x9d material). An adhesive which is responsive to temperature change (usually one which melts when the temperature is raised) is provided between the sleeve and the spliced fibres. Sleeves themselves are accurately made to predetermined dimensions such that, upon the application of heat, they shrink to a smaller size thereby applying a firm grip around the end portions of the optical fibres to either side of the splice. Additional reinforcement may be provided in the form of a needle or rod, typically of stainless steel, housed within the splice support sleeve and pressed against the optical fiber as the sleeve shrinks. The adhesive may be provided as an inner element of adhesive sealant material which forms a liner within the sleeve to ensure intimate contact with the optical fiber and, in particular, increase the tensile strength of the bond between the sleeve and the fiber. The adhesive also acts as a sealant to encapsulate the ends of the fibres and protect them from moisture and dust.
In order to effect heat shrinking of a sleeve onto a splice it is positioned within a suitable appliance which heats it to a temperature sufficient to bring the sleeve and the adhesive to the temperature range at which shrinkage takes place. This may be as high as 250xc2x0 C. At such elevated temperatures, indeed above about 50xc2x0 C., the heat shrink material and the adhesive are soft and plastic and it is therefore necessary for the sleeve (and the splice within it) to be allowed to cool for some considerable time, a matter of several minutes after heating has been discontinued, before the reinforced splice can be safely handled and positioned, for example, in a splice holder forming part of the installation. If the splice support sleeve is disturbed before it has cooled sufficiently it may be deformed upon fitting to the splice holder as a result of which the optical fiber within it may be flexed or bent, and will be placed under stress. This stress may, over time (with thermal cycling and other variations) result in breakage of the fiber at the splice. This may occur not immediately, but several days, weeks or even months after the splice was made. This is a considerable disadvantage, resulting in the need for service personnel to attend the site to remake a failed splice, as well as the downtime in the equipment the operation of which is affected by the failed splice.
The present invention seeks to address this problem by providing means by which a splice can be made and reinforced with more accurate information on the temperature of the splice support sleeve and the hotmelt adhesive thereby allowing sufficient time for the sleeve to cool to a temperature at which it can be handled safely without risk of damage to the spliced fiber, but at the same time avoiding unnecessary delays in a procedure which is already somewhat time consuming.
A method and device for reinforcing an optical fiber splice are disclosed in Japanese Patent Application JP 62-021107 A (NTT). In this known method, pressure is applied on the reinforcing member while it is heated and the temperature on the heater surface is monitored. The means by which this monitoring is carried out are not disclosed.
According to the present invention, there is provided a support for an optical fiber splice, the support comprising an elongate support sleeve for protecting and supporting an optical fiber junction point, wherein the application of the support involves attainment of an elevated temperature, characterised by an element the colour of which changes at or in the region of a selected temperature to provide a visual indication of attainment of the selected temperature or temperature range whereby to assist in determining effective completion of a step in the application process.
In the specific application envisaged the attainment of the selected temperature or temperature range will be approached from a higher temperature during cooling although it is possible to envisage circumstances in which the attainment of a temperature from below during heating could also be indicated using a visual temperature indicator as herein defined.
In accordance with the present invention, therefore, the means for providing a visual indication of attainment of a selected temperature or temperature range may be or may include an element the colour of which changes at or in the region of the selected temperature. Other physical properties may, of course, be employed such as the obscuration of a pattern (for example by surface tension as a fusion temperature is approached) loss of transparency or other heat-influenced physical phenomenon.
It is known, for example, that certain materials exhibit specific temperature-related phenomena such as a change in colour at fairly closely defied temperature ranges. One such material is the so-called xe2x80x9cliquid crystalxe2x80x9d which comprises certain organic substances the molecules of which have an elongate form. This elongate form causes the molecules to align themselves in certain directions for example when low-intensity magnetic fields are applied. Liquid crystals have been widely used in the production of alphanumeric displays making use of the ability of the crystal to vary its reflective or transmissive properties by varying the anisotropy of the physical properties in dependence on applied fields. When used in a digital or alphanumeric display, a thin layer of liquid crystal material is sandwiched between two sheets of glass each having a thin, transparent, coating of conductive material with the viewing side etched into character-forming segments with leads extending to the edges of the display. Voltages applied between the front and back coatings disrupt the orderly arrangement of the molecules sufficient to cause the liquid crystal to change its reflective or transmissive properties whereby to form visible characters.
One particular form of liquid crystal, namely chiral nematic (cholesteric) liquid crystals also have the property that they can reflect light having a wavelength equal to the pitch of separation of the molecules. Because the pitch of separation, assuming the molecules are aligned, is dependent upon temperature the apparent colour, that is the colour of light reflected therefrom, may also depend upon the temperature of the crystal. This makes it possible to provide an indication of a temperature or temperature range by observation of the colour of the liquid crystal. Different specific materials act to reflect different wavelengths so that a wide range of temperatures can be indicated.
It is noted that U.S. Pat. No. 5,686,153 discloses an optical temperature indicator for use in domestic appliances, such as hairdryers and coffee makers. This known temperature indicator comprises a liquid crystalline material.
In the present invention the said means for providing a visual indication of attainment of a selected temperature may comprise or include an element of liquid crystal material, although other suitable materials may be used. Such material may be present as a separate element located within a body of the support (such as a sleeve) in such a way as to be visible from the outside thereof. Suitable materials are commercially available and have known temperature ranges. The man skilled in this art would be able to identify and select an appropriate choice of material from among those commercially available and the materials themselves will not be described in more detail, although examples are given below of two such commercially available materials.
For this purpose at least a part of the said body may be transparent to allow the said temperature indicator means to be visible therethrough.
Alternatively the said temperature indicator means may comprise a temperature-sensitive coating on or on a part of the body of the support itself.
If the said body is an elongate sleeve the said coating may be formed on the outside or the inside surface thereof or incorporated within the body such as to be visible at the surface. When utilising a known structure comprising a sleeve with an internal reinforcing rod, the said coating may be formed on the reinforcing rod.
In embodiments having an elongate sleeve and a heat fusing adhesive sealant sleeve-like element within for bonding to the spliced fiber at a temperature above the said selected temperature, the temperature-indicating material may be present as a coating on or inclusion in the said adhesive sleeve.
The present invention also comprehends a method of splicing optical fibres, comprising the steps of fitting a tubular support over one fiber, fusing the fibres so as to form a splice, positioning the support over the splice, heating the support to a predetermined temperature, and allowing the support to cool, which method is characterised by the steps of determining the attainment of a selected temperature by observing an element of the support the colour of which changes at or in the region of the selected temperature and maintaining the support undisturbed until occurrence of an observable change in the colour of the element.